Various forms of preforms have been known heretofore, and a number of processes are also known already for forming such preforms. For example, Japanese Patent Application Kokoku (Post-Exam. Publn.) No. 62-780 discloses a preform which comprises a glass short fiber mat and a glass fiber woven fabric bonded therewith through the heat-melting ability of a heat-meltable short fiber web provided therebetween. Japanese Patent Application Kokai (Laid-open) No. 63-152637 discloses a preform which has a lamination structure of a reinforcing textile base material containing a thermoplastic polymer adhered to the warp or the weft or both, the textile base materials adjacent to each other being bonded with each other by said polymer to form an integral body; and a preform which has an alternate lamination structure of said reinforcing textile base material and another reinforcing fiber base material containing no thermoplastic polymer adhered thereto, said base material and said another base material adjacent to each other being bonded with each other by said polymer to form an integral body.
However, the former preform, namely the one disclosed by Japanese Patent Application Kokoku No. 62-780, tends to be bulky since it is formed by a combination of glass short fiber mat and glass fiber woven fabric, and hardly gives a preform for forming fiber reinforced plastics having a high strength in one direction. On the other hand the latter preforms, namely those disclosed by Japanese Patent Application Kokai No. 63-152,637, being a preform material mainly for use in resin injection molding, give a reinforcing fiber content of only about 25-35% by weight and even in the vacuum resin injection process the content is only 40-50% by weight, a higher content of reinforcing fiber leading to difficulty in molding. Also, with regard to orientation of fiber, since the preform is composed of laminated reinforcing textile base materials, a preform material for forming articles having a high strength in one direction alone cannot be obtained. Since the content of reinforcing fiber cannot be made sufficiently high in such molding processes mentioned above, there is a limit in obtaining reinforced plastic formed articles having high strength overall.
To increase the content of continuous reinforcing fiber such as glass fiber in forming fiber reinforced plastics, there is known as the filament winding method a process which comprises winding a resin-impregnated glass fiber on a core followed by heat curing. Such a method, however, gives formed articles whose shapes are inevitably determined by the shape of the core and which are limited to those from which the core can be taken away.
The present inventors, in order to form a preform in which continuous fiber is arranged in a state of tension and to obtain a fiber reinforced plastic having high strength and high rigidity in the direction of the arranged continuous fiber by using the preform, tried a method of forming a preform by winding, with simultaneous traversing, in layers a continuous fiber strand, e.g. glass fiber roving, in a state of tension round a rotating mandrel. However, forming a preform merely by winding a roving around a mandrel presents a problem in that the winding layers of the preform will be disordered at the time of detaching the preform from the mandrel after its formation or in handling the detached preform.
The object of the present invention is to provide a preform for forming fiber reinforced plastics in which the bulk density is suppressed and the fiber content is increased by so-called filament winding method and which at the same time has high strength and high rigidity in the direction of arrangement of continuous fiber and undergoes no disorder of winding layers at the time of detaching the preform from the core or in forming fiber reinforced plastics by using the preform.